1. Field of the Invention
The present invention relates to a method and apparatus for forming fiber reinforced composite parts.
2. Description of the Related Art
Fiber-reinforced composite structures, such as carbon-carbon composites for example, are widely used as friction materials for heavy-duty brakes in automobiles, trucks, and aircraft. This is because they exhibit high thermal conductivity, large heat capacity, and excellent friction and wear characteristics and thus can provide excellent performance.
However, past manufacturing processes for producing these fiber-reinforced composite structures were often lengthy undertakings, requiring months to fabricate a single part. In one example, a typical fiber-reinforced composite part was prepared by a non-woven process that involved needle-punching layers of carbon fibers to form a preform, a slow, time-consuming process. When two or more layers of fibers are needle punched together by metal needles having barbs on one end, the barbs commingle fibers from a particular layer into successive layers. The commingled fibers essentially stitch the layers of fiber together. This non-woven technology achieved preform densities on the order of about 0.5 g/cc. To obtain a final composite part, the preform was subsequently infiltrated with a matrix binder material via a chemical vapor deposition (“CVD”) or chemical vapor infiltration (“CVI”) process, for example. CVD and CVI are used interchangeably for the purposes of the present application.
In another process, a preform was prepared by building up successive layers of pre-impregnated carbon fiber fabric. Tows (the term “tow” is used hereinafter to refer to a strand of continuous filaments) of carbon fiber were woven into a two-dimensional fabric, and thereupon dipped into a liquid bath to impregnate the fabric with a liquid resin. The resin-impregnated fabric was then pulled between rollers to form a sheet of pre-impregnated carbon-fiber fabric. After impregnation, the fabric was dried and b-staged under low heat. A plurality of desired shapes were then cut out of the sheet material and stacked within a mold, and subsequently cured using heat and pressure to obtain the desired composite part. In order to produce a carbon-carbon component, the composite part is carbonized which creates internal porosity. Multiple infiltration cycles using CVD or resin were required to achieve final density of the composite part. For this reason, it often took a term of several months to obtain a final product, causing the product to be extremely expensive. Further, much material was wasted in order to obtain the final product.
Several processes have been developed in order to reduce overall processing time needed to manufacture a fiber reinforced composite part. One process, a “random-fiber process”, uses entirely tow material. Somewhat similar to the pre-impregnating method described above, in the random-fiber process a continuous tow of fiber is dipped through a resin bath, dried, and then chopped to a desired length. The resin coated chopped fibers are then placed into a mold and cured using heat and pressure. However, the steps of impregnating the continuous tow are performed separately from the molding and curing required to create the composite part, thereby extending the “process cycle” of manufacturing the composite part.
Another method involves a molding compound process whereby chopped fibrous material are mixed with a resin so as to form a continuous sheet of mixed material. A plurality of desired shapes are then cut out of the sheet material and stacked within a mold, and subsequently cured using heat and pressure to obtain the desired composite part. Again, this process requires extensive time and wastes material in order to obtain the final product.
A further process developed to shorten the manufacturing time involves using a liquid slurry to mix the fibrous material with a resin powder, as illustrated in U.S. Pat. No. 5,744,075 to Klett et al. However, the fibrous material needs to be chopped into small pieces (on the order of ¼ to ½ inch (about 0.6–1.3 cm)) so as to attain a uniform mix with the resin powder in the slurry. Thus, longer chopped fibers (1–1½ inches (about 2.5–3.8 cm)) do not work well in this liquid slurry method, since a uniform dispersion of fibrous material and resin powder in the slurry cannot be attained with the longer chopped fiber lengths. The longer fibers tended to “ball-up” during mixing with the powdered resin and during deposition into the mold, making it difficult to obtain a uniform end product. Moreover, this “balling effect” directly contributed to the “loftiness” of the preform, a disadvantageous result of the water slurry method since a lofty preform was difficult to control within the mold. Additionally, an excess step of drying the preform was required (i.e., removing the water from the preform in the heating step is required before pressing the materials into a composite part).
Recent developments have introduced a method and apparatus that combines chopped fibers and a powdered resin utilizing a dry-blending process. Such a dry-blending process and apparatus 100 is illustrated in the rough schematic diagram of FIG. 1. Apparatus 100 includes a first lower enclosure 101 connected to a second upper enclosure 102 via a neck portion 119. First enclosure 101 has an adjuster 120 connected thereto which houses compressed air lines 121 and 124 for feeding air jets 122. Second enclosure 102 houses a screen 126, and has a funnel 132 and vacuum line 135 connected thereto.
In FIG. 1, chopped tow 115 is loaded into first enclosure 101, where air jets 122 feed compressed air into the chopped tow 115 within first enclosure 101. The compressed air fed via compressed air lines 121 and air jets 122 enters below the level of chopped tow in first enclosure 101. This compressed air forces the chopped tow 115 into upper portion 117 of first enclosure 101 such that the individual fibers of the chopped tow 115 are entrained in air and further broken-up (defibrillated) into smaller strands or filaments 118. Adjuster 120 maintains the compressed air jets 122 at a level equal to or below the chopped tow 115 within first enclosure 101.
The broken-up fibers 118 entrained in air in the upper section 117 are then forced through neck portion 119 into a second enclosure 102, whereby they are mixed with a powdered resin 130 fed through at funnel 132 of second enclosure 102. The powder resin 130 mixes with the broken-up fibers in a powder and fiber mixing region 140, whereupon the “mixed materials” settle at the bottom of second enclosure 102 to form a layer which constitutes the building-up of a preform 125. The mixed materials fall due to a vacuum 135 being applied to the bottom of second enclosure 102 which removes the bulk of the air volume in second enclosure 102, thereby allowing the mixed materials to fall and condense at the bottom of second enclosure 102 on top of screen 126.
The “dry-blending” apparatus of FIG. 1 provides a medium for mixing the powder 130 with the fibrous material (chopped tow 115) to attain a uniform mixture of the binder material with the fibrous material. However, in the apparatus 100 of FIG. 1, the proportions of chopped fiber and binder material have to be first individually weighed out to obtain the proper proportions, before being loaded in enclosures 101 and 102 to be mixed in mixing region 140. Further, apparatus 100 of FIG. 1 is limited to a single-batch process, i.e., to make one final fiber-reinforced composite part, the individual proportions for each fibrous material and binder material have to be weighed and added individually for each preform made.
Yet a further process to shorten the manufacturing cycle time of a composite part is illustrated in U.S. Pat. No. 5,236,639 to Sakagami et al. The objective of this process is to provide excess carbon material to fill pores in the matrix material during subsequent curing and carbonization steps, thus producing a carbon-carbon composite material that requires no repetition of production steps including any further densification of the composite material. This involves mechanically mixing a matrix carbon material and carbon fibers in proportions that are determined on the basis of the carbonization ratio of the matrix material and on the basis of the desired ratio of fibers to be contained in a resultant end product. However, such a process requires the use of excess carbon matrix material, a curing step under pressure after formation of an intermediate-formed part such as a preform or mold, and also requires subsequent carbonization and graphitization of the cured intermediate part, both under pressure, to obtain the final fully-densified composite part. Of course, no further production steps are required or repeated, including densification of the composite material. However, it is costly and time consuming to perform the curing, carbonization and graphitization all under pressure.
Therefore, what is desired is a method and apparatus which would feed, blend, and deposit various lengths of chopped fibrous and binder materials into a mold of a desired final shape, wherein the raw fibrous materials and binder materials are combined in a single step, followed by consolidation of the materials. The resultant preform would not require any curing or carbonization under pressure during the follow-on heating processes to manufacture the final composite part. Such a method and apparatus would provide fiber-reinforced composite parts with densities that are higher than achieved with current technologies, and would decrease overall cycle time to a finished composite part. The method can be used to provide an intermediate preform product that is subsequently stabilized, carbonized, optionally heat treated, densified, and final heat treated to provide a carbon-carbon composite material.